1. Field of the Invention
The present invention relates to a fixed constant velocity joint, and a vehicle steering apparatus including this fixed constant velocity joint, and in particular, relates to a fixed constant velocity joint appropriate for an application where a rotational backlash should be avoided.
2. Description of the Related Art
A constant velocity joint is divided roughly into a fixed type which allows only an angular displacement between input and output shafts, and a slide type which allows both an angular displacement and an axial displacement, and they are individually selected according to their applications and operation conditions.
As the fixed constant velocity joint, Rzeppa type (abbreviated as “BJ” hereafter) and undercut free type (abbreviated as “UJ” hereafter) are widely known.
Both the BJ and the UJ include an outer race having a plurality of curved track grooves on an inner peripheral surface thereof, an inner race having a plurality of curved track grooves on an outer peripheral surface thereof, balls incorporated between the track grooves on the inner race and the outer race, and a cage for holding the balls. The track center of the outer race is displaced from the spherical center of the inner periphery of the outer race, and the track center of the inner race is displaced from the spherical center of the outer periphery of the inner race. Both are displaced by the same distance, but in the opposite directions in the axial direction. As a result, the ball track including the track groove on the outer race and the track groove on the inner race has a wedge shape widening toward the inner side or the opening side of the outer race. The entire part of the individual track grooves is curved with the outer race track center or the inner race track center as its center in the BJ. On the other hand, one end of the individual track grooves is straight in the axial direction in the UJ.
A clearance exists between the track grooves of the outer race and the track grooves of the inner race from the necessity of functionality and manufacturing in these fixed constant velocity joints. This clearance (track clearance) is a travel in the radial or the axial direction of the inner race or the outer race which is not fixed while the other one of the inner race or the outer race is fixed in a neutral state of the joint, and is called as a track clearance in the radial direction or a track clearance in the axial direction depending on the direction of the travel.
The magnitude of the track clearance largely affects a backlash in the circumferential direction (a rotational backlash) between the inner race and the outer race (the rotational backlash increases as the track clearance increases). Since the track clearance is indispensable in the fixed constant velocity joint as described above, and thus, the generation of the rotational backlash more than a certain amount is unavoidable, the fixed constant velocity joint of this type has not been used generally for an application such as a steering apparatus for a vehicle where a backlash should be avoided.